This invention relates to a method of flame treating polymeric substrates to modify the surface properties of the substrate and to articles treated by the method.
Flame treating is used to improve the wetting and adhesion properties of polymer film surfaces in general and of polyolefin film surfaces in particular. The most wettable surface-modified polymer films usually have optimal adhesion properties in a variety of practical applications. The enhanced wetting property results in improved coatability and adhesion of materials such as pressure-sensitive adhesives, primers and low-adhesion release coatings. Enhanced wetting properties are particularly useful in coating water-borne solutions at all film speeds and in coating solvent-borne materials at high coating speeds.
Flame treaters ordinarily use premixed flames, i.e., the fuel and oxidizer are thoroughly mixed prior to combustion and the rate of combustion is controlled by the rate of chemical reaction that occurs in the flame. In a premixed flame, the luminous region is that portion of the flame where the rise in temperature is the greatest and where much of the reaction and heat release occur. During a flame-treating process, one side of a polymer film is passed in close proximity to a flame while the other side of the polymer surface generally passes over a cooled support, e.g., a cooled drum, to minimize heat distortion.
Flames are commonly described in terms of two characteristics: the flame power and the molar ratio of oxidizer to fuel. The flame power is the product of the volume of fuel burned per unit time and the heat content of the fuel. Typical units for the flame power are W or Btu/hr. In fame treating, the flame power can be normalized to account for the dimensions of the burner, leading to units such as W/cm2 or Btu/hr-in2. The exact ratio of oxidizer to fuel needed for complete combustion is known as the stoichiometric ratio. For example, the exact amount of dry air necessary for the complete combustion of methane is 9.55 volumes per volume of methane; so the stoichiometric ratio for an air:methane flame is 9.55:1. The equivalence ratio is defined as the stoichiometric oxidizer:fuel ratio divided by the actual oxidizer:fuel ratio. For fuel-lean, or oxidizing, flames, there is more than the stoichiometric amount of oxidizer and so the equivalence ratio is less than one. For oxidizer:fuel mixtures at the stoichiometric ratio, the equivalence ratio is equal to one. For fuel-rich systems, the equivalence ratio is greater than one.
Virtually all industrial flame treaters use a premixed laminar (as opposed to turbulent) flame with air as the oxidizer and a gaseous hydrocarbon as a fuel. Typical hydrocarbon fuels include hydrogen, natural gas, methane, ethane, propane, butane, ethylene, liquefied petroleum gas, acetylene, or blends thereof, and city gas, which is often composed of a mixture of carbon dioxide, carbon monoxide, hydrogen, methane, and nitrogen. Halogen and halogen-containing compounds have also been disclosed as oxidizer:fuel mixture additives to increase the adhesivity of polyolefin films to subsequent coatings.
Recently, hydrocarbon flames enriched with fuel and oxidizer substitutes that contain either or both oxygen and nitrogen atoms or silicon atoms have demonstrated substantial increases in wetting values on polymer films relative to a non-enriched flame process.
Improved adhesion of metal coatings to polymeric substrates has been demonstrated on different types of substrates, some polymerized from a mixture containing monomer and a sulfur-containing compound such as barium sulfate, and others made by exposing a polymeric surface to a vacuum glow discharge or plasma created with a sulfur-containing gas such as hydrogen sulfide.
Because of the benefit that the increased wettability of polymer surfaces has on the coating industry, there is an ongoing need for processes that improve the wettability of polymer films. In addition; because metal coatings are desired on a wide variety of polymeric surfaces, there is an ongoing need for processes that enhance the adhesion of metal to polymeric surfaces.
The present invention provides a method of modifying the surface of a polymeric substrate, e.g., to improve the wettability of the polymer substrate surface or to improve the adhesion of the polymeric substrate to a subsequently applied metal coating layer. Increased affinity occurs between a polymeric substrate and a metal layer that does not have an increased affinity to the surface of the polymeric substrate when it is further oxidized. The method of the invention comprises exposing the substrate to a flame that is supported by an oxidizer and fuel mixture that includes at least one sulfur-containing compound that functions as a fuel substitute.
In one preferred embodiment, the oxidizer and fuel mixture has an equivalence ratio that is either fuel-lean or stoichiometric and the resulting flame-treated polymeric film exhibits improved wettability. Surprisingly, the treated surface of the polymeric film generally has at least one oxidized sulfur-containing chemical group and at least one nitrogen-containing chemical group. These films provide improved wettability.
In another preferred embodiment, the fuel and oxidizer mixture has an equivalence ratio that is fuel-rich and the resulting flame-treated polymeric film exhibits improved adhesion to a subsequently applied metal coating. The treated surface of the polymeric film generally has at least one oxidized sulfur-containing chemical group and at least one unoxidized sulfur-containing chemical group.
This invention also provides novel articles. One article provided by this invention is a polymeric surface having at least one oxidized sulfur-containing chemical group and at least one nitrogen-containing chemical group from the group consisting of nitroso and nitrosoamine. Another article provided is a polymeric surface having at least one oxidized sulfur-containing chemical group and at least one unoxidized sulfur-containing chemical group.
Large increases in wettability as measured by ASTM D-2578-84 Wetting Test, e.g., greater than 16 mJ/m2 over that obtained with conventional flame-treating processes using natural gas, have been observed in a preferred polymeric substrate treated according to this invention. In addition, significant increases in the adhesion of polymeric substrate surfaces to a metal, e.g., over 50 percent increase for polypropylene to silver, have been observed.